1. Field of the Invention
The present invention relates to a secondary battery having a safety valve.
2. Description of the Related Art
In recent years, in consideration of environmental issues, secondary batteries represented by lithium-ion batteries have been applied not only to mobile objects such as hybrid electric vehicles (HEVs), electric vehicles (EVs), forklifts, and excavators but also to industrial purposes such as uninterruptible power supplies (UPSs) and storages of electric power generated by solar power generation. As the usage of the secondary batteries is spread, it is sought to increase the capacity and the energy density of the secondary batteries.
In addition to increasing the performance of the secondary batteries, increasing the safety of the secondary batteries is also an important issue. Examples of the secondary batteries include nickel-cadmium batteries, nickel-hydrogen batteries, and lithium-ion batteries. Among the currently existing secondary batteries, the lithium-ion secondary batteries are particularly suitable for increasing the energy density, and are currently actively developed.
The nickel-hydrogen batteries and the lithium-ion secondary batteries include, as their main constituent elements, a negative electrode having a negative current collecting member and a negative active material layer held on a surface of the negative current collecting member, a separator holding an electrolyte, and a positive electrode having a positive current collecting member and a positive active material layer held on a surface of the positive current collecting member. The nickel-hydrogen batteries include a nickel oxide in the positive active material layer of the positive electrode and a hydrogen-storing alloy in the negative active material layer of the negative electrode. The lithium-ion secondary batteries include a lithium metal oxide in the positive active material of the positive electrode and a carbon material such as graphite in the negative active material layer of the negative electrode. From the viewpoint of the battery structure, the secondary batteries are roughly classified into those having a cylindrical structure, in which an elongated negative electrode, an elongated separator, and an elongated positive electrode are sequentially layered to make layered structure and the layers are wound into a swirling shape, and those having a layered structure, in which a rectangular negative electrode, a rectangular separator, and a rectangular positive electrode are layered. In general, the layered batteries, in which the rectangular negative electrode, the rectangular separator, and the rectangular positive electrode are layered, are more suitable for increasing the energy density per volume than the batteries having a cylindrical structure, which include a greater volume of space that is not relevant to power generation such as an axial core for winding up the elongated negative electrode, the elongated separator, and the elongated positive electrode. Because the layered batteries require no axial core for wind-up and readily allow a positive output terminal and a negative output terminal to be disposed on an identical surface of a battery case, such batteries can reduce a volume of space including the parts that does not contribute to power generation. The battery case of such secondary batteries includes a case body receiving an electrode group and a lid plate having a positive output terminal member and a negative output terminal member and tightly sealing the case body.
When the conventional secondary batteries are assembled, the output positive terminal member and the output negative terminal member are connected to the electrode group including the negative electrode, the separator, and the positive electrode before the case body is sealed with the lid plate. The positive output terminal member and the negative output terminal member each include a terminal body and a terminal base portion formed at the base of the terminal body. A portion of each output terminal member that is exposed to the outside of the battery case is referred to as the terminal portion, and a portion of each output terminal member that is received inside the battery case is referred to as the terminal base portion. Generally, the terminal base portion of each output terminal member is electrically connected to the electrode group. After an assembly with the lid plate into which the positive output terminal member and the negative output terminal member and the electrode group are assembled is received in the case body, the lid plate is attached to an opening portion of the case body via an insulating member. After the assembly with the lid plate is inserted into the opening portion of the case body, the lid plate and the case body are sealed against each other. In the battery case, the separator, the positive electrode, and the negative electrode, which are constituent elements of the battery assembly, are impregnated with an electrolyte. In order to ensure safety, the battery case is provided with a safety mechanism such as a safety valve. If a rupture, an ignition, or the like occurs in batteries with a high energy density and a large capacity because of wrong use such as overcharging or a short circuit due to mixing of foreign matter, greater damage may be caused compared to the conventional batteries.
Japanese Patent Application Publication No. 2002-8616, for example, discloses a safety valve, which is disposed at a lid plate to which a positive terminal member and a negative terminal member are attached. Japanese Patent No. 3573295 proposes a safety valve disposed on a side surface of a battery case. Japanese Patent No. 3573295 also discloses that a gas discharge hole (safety valve) is disposed at a shadow area obtained by projecting a wound area of a swirling electrode group onto a side wall of a battery case to allow produced gas to be smoothly discharged to the outside of the battery through the gas discharge hole along the winding axis, in order that the produced gas can be smoothly discharged to the outside of the battery case when the safety valve is opened. Japanese Patent No. 4233671 discloses a cylindrical battery in which an electrode column is disposed at the center of a circular sealing plate, a plurality of explosion protection valves are provided at the sealing plate around the electrode column, and a joint surface for a lead is provided on the outer peripheral surface of the electrode column at a circumferential position that is not in the directions in which the plurality of explosion protection valves are disposed so that the plurality of explosion protection valves and the lead are disposed not to overlap with each other as viewed from the direction perpendicular to the sealing plate.
A battery with a large capacity discharges a large amount of gas when ruptured, and thus requires a safety valve with as large an area as possible. If the area of the safety valve is too small for the battery capacity, the amount of gas to be discharged to the outside of the battery case is small for the amount of generated gas. Therefore, the internal pressure of the battery case may not stop rising even if the safety valve is actuated, which may lead to a breakage of the battery case itself. In addition, it is necessary to determine the arrangement of the electrode group including the positive electrode, the negative electrode, and the separator, components to be energized such as terminals, and the safety valve with consideration not to hinder the flow of generated gas inside the battery case to cause a rise in internal pressure. As a breakage occurs at a higher internal pressure, a greater influence is exerted on the surrounding environment by the impact of the breakage or the like. Hence, it is necessary to increase the area of the safety valve and to make the gas flow inside the battery case smoother as the battery capacity becomes larger.
Moreover, in order to suppress a rise in temperature during electrical discharge of a battery with a large capacity, it is necessary to increase the size of the components to be energized such as a positive output terminal member and a negative output terminal member. This is because a battery with a larger capacity discharges a higher current, and therefore produces more heat at the positive output terminal member and the negative output terminal member to reduce the margin between the allowable safe temperature of the battery and the battery temperature unless the electrical resistance of the terminal members is reduced. Thus, the safety valve and the terminal members should be increased in size along with an increase in capacity of the secondary battery.
In order to enhance the energy density, it is necessary to reduce the volume of the battery as much as possible, and to reduce the size of components other than power generating elements, namely, the safety valve and the components to be energized such as the terminal members. Thus, increasing both the capacity and the energy density of a battery involves contradictory requirements for the size of the safety valve and the components to be energized such as the terminal members. In the case of a battery with a large capacity of 100 Ah or more, in particular, the amount of energy stored in the battery is large, and thus it is desired that the battery should be compact and should have as large a safety valve as possible.
In order to enhance the energy density, it is desirable to provide the safety valve and the positive output terminal member and the negative output terminal member on the same surface of the battery case as with the safety valve disclosed in Japanese Patent Application Publication No. 2002-8616. In order to suppress generation of heat due to the energization resistance along with an increase in battery capacity, however, it is necessary to increase the width of the positive output terminal member and the negative output terminal member and tabs formed on the metal current collecting members to which an active material layer is applied and electrically connected to the terminal members. In the secondary battery disclosed in Japanese Patent Application Publication No. 2002-8616, however, no consideration is given to the geometry of the components to be energized such as the output terminal members and the tabs connected to the output terminal members, and no solution is proposed to the issue that generation of heat from the components to be energized such as the terminals and the tabs due to an increase in battery capacity should be suppressed.
Therefore, increasing the size of the safety valve and increasing the width of the output terminal members and the tabs along with an increase in battery capacity may result in the tabs and the output terminal members being extended to a location directly below the safety valve, which may hinder smooth discharge of generated gas during an abnormality.
Meanwhile, if the safety valve is disposed on a side surface of the battery case to allow a smooth flow of gas produced during an abnormality as in the secondary battery disclosed in Japanese Patent No. 3573295, it is necessary to form a hole or the like in a portion of the battery case other than the lid plate on which the positive output terminal member and the negative output terminal member are disposed, which increases the cost. It is also necessary to provide a space for releasing gas or a space for wiring outside a wall surface of the battery case on which the safety valve is disposed and a wall surface of the battery case on which the output terminal members are disposed. In the secondary battery disclosed in Japanese Patent No. 3573295, however, the safety valve and the terminal members for external output are disposed at different wall surfaces of the battery case. Therefore, it is necessary to provide a free space around two wall surfaces of the battery case, which significantly restricts installation of the battery or the like compared to the case where the safety valve and the output terminal members are disposed on the same wall surface.
In the cylindrical battery disclosed in Japanese Patent No. 4233671, when the terminals for external output are disposed at both ends of the battery case, the electrode column serving as the electrode terminal is disposed at the center of each end of the battery case. Therefore, the safety valves may not be disposed at the center of an end portion of the battery case. In addition, the size of each safety valve is limited to a value obtained by subtracting the radius of the electrode column from the radius of the cylindrical battery case or less. Therefore, it is difficult to increase the diameter of the safety valves if the size of the electrode column is increased. Moreover, the output terminals are disposed at both ends of the battery case, which requires a large volume of space for elements other than the power generating elements. Thus, the energy density may not be enhanced compared to the case where the output terminals are provided on an identical wall surface of the battery case. Meanwhile, in the cylindrical battery, gas produced between two adjacent metal current collecting members is discharged along the winding axis. However, the electrode group with a layered structure uses rectangular metal current collecting members, and thus produced gas tends to spread in all directions in the battery case to fill the battery case. Therefore, when the electrode group with a layered structure is used, it is desired to more immediately discharge gas to the outside of the battery case.
Thus, it has been difficult to increase the size of the safety valve in a battery with a large capacity and a high energy density and with a layered structure in which the output terminal members and the safety valve are provided in the lid plate of the battery case.